JPH04245648A - Field effect transistor - Google Patents

Abstract

PURPOSE:To obtain a high output FET excellent in high frequency characteristics. CONSTITUTION:On a GaAs semiconductor substrate 21, the following are formed in order; a semiconductor layer 22 composed of undoped GaAs, a first semiconductor layer 23 composed of undoped InGaAs, a first channel layer 24 composed of high concentration thin-layered N<+> type InGaAs, a second semiconductor layer 25 composed of undoped InGaAs, a second channel layer 26 similar to the first channel layer 24, and a third semiconductor layer 27 composed of undoped InGaAs. Further on the third semiconductor layer 27, a fourth semiconductor layer 28 composed of undoped AlxGa1-xAs is formed. The fourth semiconductor layer 28 forms a hetero junction together with the third semiconductor layer 27, and is in Shottky contact with a gate electrode 33.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the structure of a field effect transistor (FET) which requires ultra-high speed operation.

0002

2. Description of the Related Art Conventionally, as this type of ultra-high-speed device, there is, for example, a HEMT (high electron mobility transistor) having a structure shown in FIG. An undoped GaAs layer 2 is formed on the GaAs semiconductor substrate 1, and an undoped InyGaAs layer 2 is further formed on the undoped GaAs layer 2.
A 1-y As layer 3 is formed. This undoped I
ny Ga1-y On the As layer 3, Alx Ga1-x
n-Alx Ga with donor selectively added to As
A 1-x As layer 4 is formed. Furthermore, this n-
n+ -InGa on the Alx Ga1-x As layer 4
An As layer 5 is formed, and a gate electrode 6 is formed in Schottky contact with the n-Alx Ga1-x As layer 4 exposed in the recess formed in the center. Also, n
+ -Ohmic electrodes 7 and 8 are formed on the InGaAs layer 5.

[0003] Other ultra-high-speed devices include:
For example, 2 disclosed in Japanese Patent Application Laid-Open No. 64-82677
There are also ultrafast devices with layer planar doped structures. In this device, two planar doped layers doped with impurities in a two-dimensional plane are provided with an interval within the mean free path of electrons. By using these planar doped layers for the channel layer, the speed of the device is increased.

0004

[Problems to be Solved by the Invention] In a system using an AlGaAs/InGaAs heterojunction, such as the conventional HEMT described above, electrons traveling in the InGaAs layer 3, which becomes a channel, reach the AlGaAs layer 4 located above this. This may result in real space transitions. This real space transition can be explained as follows. n-AlGaAs
An energy band shown in FIG. 7 is formed at the heterojunction between the layer 4 and the undoped InGaAs layer 3, and two-dimensional electron gas is accumulated in the hatched area shown. However, when a high electric field is applied between the drain and source and the energy of the two-dimensional electron gas increases, the electrons in the two-dimensional electron gas become n-
It transitions to the AlGaAs layer 4 side as shown by the arrow in the figure.

Generally, a high electric field is applied between the drain and source during operation, and the AlGaAs layer 4 is
Since the electron transport characteristics are inferior to those of the aAs layer 3, when this real space transition occurs, the high frequency characteristics of the FET deteriorate.

Further, in the conventional HEMT described above, a two-dimensional electron gas layer 9 generated at the heterojunction interface between the undoped InGaAs layer 3 and the n-AlGaAs layer 4 serves as a channel. The maximum current density of HEMT is determined by the upper limit of this two-dimensional electron gas concentration, but since the channel layer is two-dimensional, there is a limit to increasing the electron gas concentration. For this reason, it has not been possible to obtain a high frequency device with sufficiently high output.

On the other hand, even in the above-mentioned conventional FET having a two-layer planar-doped structure, a two-dimensional planar thin planar-doped layer is used for the channel layer, and the thickness of the planar-doped layer is about 5 to 6 angstroms. It has a thickness of several layers. Therefore, the impurity doping amount for this planar doped layer is at most about 1.times.10@13 /cm@2. For this reason, the electron concentration in the channel layer is limited even if two planar doped layers are provided, and like the conventional HEMT described above, although it is possible to increase the speed of the device, it is hindered from increasing its output. was.

[0008]

[Means for Solving the Problems] The present invention has been made to solve the above problems, and provides a first semiconductor layer made of a material with excellent electron transport properties that does not contain any impurities or contains impurities at a low concentration. Iny Ga1-y As (0≦Y≦
0.35), and a second channel layer made of a material with excellent electron transport properties that does not contain any impurities or contains a low concentration of impurities and has a crystal structure that is approximately lattice matched to the first channel layer. A semiconductor layer and a thin InyGa1-yAs layer containing a high concentration of n-type impurity having a crystal structure that is almost lattice matched to the second semiconductor layer.
(0≦Y≦0.35), and a material with excellent electron transport properties that does not contain any impurities or contains impurities at a low concentration, and has a crystal structure that is almost lattice-matched to the second channel layer. a third semiconductor layer consisting of AlX Ga1-X As (0≦X≦0. 3) and a fourth semiconductor layer consisting of the following.

[0009]

[Operation] When a low electric field is applied between the drain and source, electrons generated in the first and second channel layers containing high impurity concentrations contain no impurities located between these channel layers. The probability of the second semiconductor layer having excellent electron transport properties increases the probability that the second semiconductor layer does not contain or contains a low concentration of electrons.

When a high electric field is applied between the drain and source, electrons traveling in the channel gain energy and the first
The electrons then jump out to the first and third semiconductor layers, which sandwich the second channel layers and have excellent electron transport properties.

Furthermore, since each of the first and second channel layers has a certain thickness, they can contain impurities at a high concentration, and the channel is formed by a large amount of electrons.

0012

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows the structure of an FET according to an embodiment of the present invention, and its manufacturing method is shown in the cross-sectional views of FIG. 2. This manufacturing method will be explained below.

First, a semi-insulating GaAs semiconductor substrate 21
On top is a semiconductor layer 2 for lattice matching with this substrate 21.
2, first semiconductor layer 23, first channel layer 24, second
semiconductor layer 25, second channel layer 26, third semiconductor layer 27, fourth semiconductor layer 28, and contact layer 29
are sequentially epitaxially grown (see FIG. 2(a)). This epitaxial growth is performed by MBE (molecular beam epitaxy) or OMVPE (organic metal vapor phase epitaxy).

The semiconductor layer 22 for achieving lattice matching with the substrate 21 is made of undoped GaAs and has a thickness of 1 μm.
It is m. The first semiconductor layer 23 includes each channel layer 24,
Undoped In with better electron transport properties than 26
Consisting of y Ga1-y As (0≦Y≦0.35),
The thickness is 50 angstroms. The first channel layer 24 and the second channel layer 26 each have a size of 2×10
They are made of n+ type InyGa1-yAs (0≦Y≦0.35) doped with Si at a concentration of 18/cm3, and each has a thickness of 100 angstroms. The second semiconductor layer 25 sandwiched between the channel layers 24 and 26 is made of undoped Iny Ga1-y As (0≦Y≦
0.35) and has a thickness of 100 angstroms. The third semiconductor layer 27 on the second channel layer 26 is made of the same material as the second semiconductor layer 25 and has a thickness of 100 angstroms. The fourth semiconductor layer 28 forming a heterojunction with the third semiconductor layer 27 is made of undoped Alx Ga1-x As (0≦X≦0.3)
The thickness is 200 angstroms. The contact layer 29 on this fourth semiconductor layer 28 has a size of 4×10
It is made of n+ type InGaAs doped with donors at a concentration of 18/cm3 and has a thickness of 500 angstroms.

Here, the carrier concentration and thickness of each channel layer 24 and 26 are set to a concentration and thickness sufficient to form a quantum well, which will be described later. In addition, since the electrons in each of these channel layers 24 and 26 have energy, the electrons in each of these channel layers 24 and 26 have energy.
, 26 is present in an area slightly wider than the thickness of . Second semiconductor layer 2 sandwiched between channel layers 24 and 26
As will be described later, the thickness of each channel layer 24,
The thickness is such that the spread of the electrons generated in each of the regions 26 and 26 sufficiently overlaps each other. Further, the thickness of the third semiconductor layer 27 on the second channel layer 26 is such that this region where electrons are spread does not reach the fourth semiconductor layer 28.

In other words, the energy band near the channel of the present FET formed by the channel layers 24 and 26 has the structure shown in FIG. 3(a). Further, FIG. 2B shows a semiconductor region corresponding to this energy band. That is, the energy band diagram shows, in order from the substrate surface side located on the left side of the figure, the third semiconductor layer 27, the second channel layer 26, the second semiconductor layer 25, and the first channel layer 24.
and are drawn corresponding to the first semiconductor layer 23. Therefore, the central part of the band corresponds to the second semiconductor layer 25 sandwiched between each channel doping plane. Here, both sides of each channel layer 24, 26, which is thinned with high concentration, are sandwiched between semiconductor layers 23, 25, 27, and each channel layer 24, 26 is sandwiched between semiconductor layers 23, 25, 27.
, 26 are formed as thin as 100 angstroms. Therefore, the conduction band is bent, and a V-shaped potential is formed corresponding to each channel layer 24, 26 as shown in the figure, thereby forming two quantum wells. Note that the thickness of each channel layer 24 and 26 is 100 angstroms, but in order to form such a quantum well in the conduction band, they must be thin to a certain extent, for example, each of 200 angstroms or less. good.

Electrons in the channel are in the lowest subband EA in the ground state with a small applied electric field. However, by applying a larger electric field to gain energy, the electrons move to the subband EB located above this subband EA, and then to subbands with higher energy levels. Here, the existence probability of electrons in each subband exhibits the spread of the wave function shown in the figure, and each channel layer 24,
In each of the semiconductor layers 23 and 27 sandwiching 26, the value approaches zero at a portion slightly wider than the semiconductor region where the channel layers 24 and 26 are formed. Furthermore, the thickness of the second semiconductor layer 25 sandwiched between the channel layers 24 and 26 is such that the existence probability of electrons generated in each quantum well is highest at approximately the center of each doped surface in the ground state. The thickness is set to In other words, the thickness is set such that the wave function waveforms drawn corresponding to each quantum well in the ground state overlap each other sufficiently at approximately the center of each doped surface, as shown in the figure. In addition, undoped InGaAs on the surface side of the substrate
The thickness of the third semiconductor layer 27 is such that electrons distributed from the second channel layer 26 to the left side in the drawing do not reach the fourth semiconductor layer 28 .

Further, a gate electrode, which will be described later, is formed in Schottky contact with this fourth semiconductor layer 28, but its thickness is such that no current flows out from this gate electrode due to the tunnel effect. There is. These fourth semiconductor layer 28, third semiconductor layer 27, and second channel layer 26
, the second semiconductor layer 25, and the first channel layer 24, while satisfying the above-mentioned conditions regarding layer thickness, the total thickness thereof is sufficiently thin to satisfy the operational performance of the FET. It becomes. In addition, the uppermost contact layer 29 serves to protect the substrate surface and to serve as a drain electrode and
This is for making ohmic contact with the source electrode, and has nothing to do with the essence of the present invention.

Next, as described above, after each layer is sequentially formed on the semiconductor substrate 21, AuGe/Ni metal is formed on the uppermost contact layer 29. Then, an ohmic electrode pattern is formed using a normal photolithography technique, and a drain electrode 30 and a source electrode 31 are formed in ohmic contact with the contact layer 29 (FIG. 2(b)).
reference). Next, a gate electrode pattern is formed using a similar conventional photolithography technique. Then, using this pattern as a mask, the contact layer 29 located in the center between the drain electrode 30 and the source electrode 31 is selectively removed by etching to form a recess 32 (see FIG.
c).

Next, a Ti/Pt film is made in Schottky contact with the fourth semiconductor layer 28 exposed in this recess 32.
A gate electrode 33 made of /Au metal is formed. As a result, an FET having the structure shown in FIG. 1 is completed.

In such a structure, the drain electrode 3
When a low voltage is applied between the 0 and source electrodes 31, a low electric field is applied to the electrons in each channel layer 24, 26, and the electrons in each channel layer 24, 26 are driven as shown in subband EA in FIG. distribution according to the wave function waveform. In other words, the probability that electrons generated from the first and second channel layers 24 and 26 exist in the undoped second semiconductor layer 25 located between these channel layers 24 and 26 increases. Therefore, electrons in the channel are less affected by impurity scattering and travel through the channel at high speed.

As the voltage between the drain electrode 30 and the source electrode 31 is increased, each channel layer 2
The electric field strength in 4,26 increases. Therefore, subband E
The electrons in the channel, which were distributed in A, move to the subband EB, which has a higher energy level, due to the energy provided by this increase in electric field strength. Furthermore, when the drain-source voltage is increased, the electrons in the channel sequentially move to higher subbands, and eventually move from each V-type potential to the first semiconductor layer 23 and the second semiconductor layer sandwiching each channel layer 24, 26. It jumps out to the semiconductor layer 27 of No. 3. On this occasion,
The amount of electrons that jump out is overwhelmingly larger in the third semiconductor layer 27 located on the second channel layer 26, and the electrons mainly travel in this third semiconductor layer 27. Since the undoped semiconductor layers 23 and 27 have excellent electron transport properties, electrons travel through the channel at high speed. Therefore, the electron saturation speed does not deteriorate.

As described above, according to the present embodiment, even if highly doped channel layers 24 and 26 which are susceptible to impurity scattering are used, electrons can move at high speed over the entire range from low to high electric field strength. Run through the channel with. Therefore, this FET exhibits high frequency characteristics that are equal to or better than HEMTs, and the cutoff frequency ft and maximum oscillation frequency fmax are not degraded compared to HEMTs. Furthermore, since the electron mobility in a low electric field is improved, the value of the source parasitic resistance Rs of the FET, which is affected by the electron mobility in a low electric field, is reduced. Additionally, the noise figure of the FET is reduced over the entire operating range because the electron channel velocity is increased over the entire range of field strengths.

Furthermore, the characteristics of mutual conductance gm with respect to changes in gate voltage Vg in each conventional FET have a characteristic in which the gm value peaks at a certain gate voltage value. However, in the mutual conductance characteristics according to this embodiment, the channel traveling speed of electrons increases over the entire range of electric field strength as described above, so the peak of the gm value is maintained over a wide range of gate voltage changes. has. Therefore, according to this embodiment, the design of the FET becomes easy, and the characteristics of the resulting FET become stable, making it possible to always ensure a high gain and providing an output without distortion.

Furthermore, in the FET according to this embodiment,
The fourth semiconductor layer 28 made of AlGaAs and the second channel layer 26 are located apart from each other by a distance equal to or longer than the spread of the wave function of electrons in the channel layer 26, as described above. Therefore, unlike a conventional HEMT in which the AlGaAs layer, which has poor electron transport characteristics, and the channel layer are close to each other, deterioration of high frequency characteristics due to real space transition does not occur.

[0026] Conventionally, it was thought that a quantum well for accumulating channel electrons could only be obtained by forming a channel layer in a two-dimensional planar shape, as seen in planar-doped FETs. Therefore, the amount of impurity doped into the channel layer in a planar-doped FET is at most 1×1013 as explained in the conventional technology.
/cm2. However, according to this embodiment,
Even if the channel layer has a certain thickness, it is possible to form a quantum well as described above. Therefore, each channel layer 24, 26 of the present FET can be doped with impurities at a high concentration, and the channel is formed by a large amount of electrons. For example, the amount of impurity doped into the channel of the present FET can be as low as 8×10 13 /cm 2 per channel layer. This is because even if the doping amount per atomic layer (per 5 to 6 angstroms) is estimated as low as 5×10 12 /cm 2 , the thickness of each channel layer 24 and 26 is 100 angstroms. Therefore, in this FET, a much larger amount of channel electrons can be obtained than in a planar-doped FET, and a larger drain current can be obtained. In addition, conventional HEMTs whose current driving ability is limited by the upper limit of the two-dimensional electron gas concentration
Even compared to this, a far superior current drive capability can be obtained.

Furthermore, since the gate electrode 33 forms a Schottky contact with the fourth semiconductor layer 28 made of undoped AlGaAs, the Schottky barrier becomes high. Therefore, it becomes possible to operate the device under high bias conditions, and this also improves the output characteristics.

Therefore, the FET according to this embodiment is effective when applied to the basic structure of an ultra-high frequency, high output, and low noise element.

In the above embodiment, the first, second and third semiconductor layers 23, 25 and 27 sandwiching the channel layers 24 and 26 are made of undoped InGaAs, but they are not necessarily limited to this material. Not done. It has a crystal structure that is approximately lattice matched to each channel layer 24, 26,
For example, undoped GaAs, which has excellent electron transport properties, may be used, and the same effect as in the above embodiment can be obtained. Furthermore, since the electrons forming the channel mainly travel through the second semiconductor layer 25 and the third semiconductor layer 27, the first semiconductor layer 23 does not necessarily have to be made of the same material as these semiconductor layers 25 and 27. Semiconductor layer 22 and first channel layer 24
It is sufficient if it has a crystal structure that is approximately lattice-matched to .

[0030] In addition, in the above description of the embodiment, two highly concentrated thin channel layers are provided, but it is also possible to provide only one such channel layer. The structure of such a single-layer channel FET is shown, for example, in the cross-sectional view of FIG.

A semiconductor layer 42, a semiconductor layer 43, a channel layer 44, a semiconductor layer 45, a semiconductor layer 46, and a contact layer 47 are epitaxially grown on a semi-insulating GaAs semiconductor substrate 41 in this order. The semiconductor layer 42 corresponds to the semiconductor layer 22 for achieving lattice matching with the substrate 21 in the above embodiment, similarly, the semiconductor layer 43 corresponds to the first semiconductor layer 23, and the channel layer 44 corresponds to the first channel layer 24, the semiconductor The layer 45 corresponds to the third semiconductor layer 27 , the semiconductor layer 46 corresponds to the fourth semiconductor layer 28 , and the contact layer 47 corresponds to the contact layer 29 . That is, each of these layers shown in FIG. 4 is made of the same material and has the same thickness as the corresponding layer in the above embodiment. Further, the drain electrode 48, the source electrode 49, and the gate electrode 50 are also formed corresponding to each electrode in the above embodiment.

FIG. 5 shows the energy band near the channel of such a single layer channel structure. The left side of the figure is the substrate surface side, and the center part corresponds to the channel doping surface. Highly concentrated thin channel layer 4
4 is sandwiched between semiconductor layers 43 and 45, and the thickness thereof is thin, so that the conduction band is bent, a V-shaped potential is formed, and the illustrated quantum well is formed. Electrons in this quantum well are distributed in each subband EA
, EB and EC are distributed as shown in the wave function waveform shown in the figure. Therefore, the drain-source voltage is low,
When the electric field strength applied to the channel layer 44 is low,
Electrons in the channel reside in the lowest energy level subband EA. As can be understood from the waveform drawn in the subband EA, the probability of the existence of electrons in the channel layer 44 is
The peak appears approximately at the center of the area. Therefore, when the electric field strength in the channel is low, electrons traveling in the channel are greatly affected by impurity scattering, and their speed decreases.

However, the present FET according to the above embodiment
In this case, electrons in the channel in a low electric field are likely to exist in the undoped second semiconductor layer 25 between the first and second channel layers 24 and 26, as described above. Therefore, in the two-layer channel FET according to this embodiment, the electron velocity is sufficiently high even in a low electric field, and the electron mobility is maintained high over the entire range of electric field strength. Therefore, the present FET according to the above embodiment has better high frequency characteristics and mutual conductance gm characteristics, and also has better noise performance. Furthermore, the source parasitic resistance Rs is reduced.

Note that when the electric field strength applied to the channel is high, even in a single-layer channel FET, the electron mobility is equivalent to or better than that of a HEMT. That is, when the electric field strength increases even in a single-layer channel FET, the electrons in the channel layer 44 sequentially shift to subbands EB and EC with higher energy levels, and eventually the electrons jump out of the quantum well and form an undoped semiconductor. This is because it runs on layers 43 and 45.

Furthermore, since such a single-layer channel FET has only one channel layer 44 which is thinned with high concentration, the amount of electrons forming the channel is smaller than that of the two-layer channel FET according to this embodiment. There aren't many. Therefore, the FET according to this embodiment has a better current drive ability, and the FET can achieve higher output.

[0036]

[Effects of the Invention] As explained above, according to the present invention, when a low electric field is applied between the drain and the source,
Electrons generated in each of the first and second channel layers containing impurities at a high concentration are transferred to a second semiconductor layer with excellent electron transport properties that does not contain any impurities or contains impurities at a low concentration located between these channel layers. The probability of it existing increases. Furthermore, when a high electric field is applied between the drain and the source, electrons traveling in the channel gain energy, and the first and third channel layers, which have excellent electron transport properties, sandwich the first and second channel layers. jumps out to each semiconductor layer. Also, the first and second
Since each channel layer has a certain thickness, it can contain impurities at a high concentration, and the channel is formed by a large amount of electrons.

Therefore, it is possible to provide an FET with excellent current drive ability without reducing the speed of electrons traveling in the channel.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing the structure of an FET according to an embodiment of the present invention.

FIG. 2 is a process cross-sectional view showing a method of manufacturing the FET according to the embodiment shown in FIG. 1;

FIG. 3 is a diagram showing the energy band structure near the channel in the FET according to the present example.

FIG. 4 is a cross-sectional view showing the structure of a single-layer channel FET considered as a modification of this embodiment.

FIG. 5 is a diagram showing an energy band structure near the channel in the single-layer channel FET shown in FIG. 4;

FIG. 6 is a cross-sectional view showing the structure of a conventional HEMT.

FIG. 7 is an energy band diagram for explaining real space transition in a conventional AlGaAs/InGaAs heterojunction.

[Explanation of symbols]

21... Semi-insulating GaAs semiconductor substrate 22... Semiconductor layer (undoped GaAs) 23... First semiconductor layer (undoped Iny Ga1-y As) 24... First channel layer (Si-doped Iny Ga1-
yAs) 25...Second semiconductor layer (undoped InyGa1-y
As) 26...Second channel layer (Si-doped Iny Ga1-
y As) 27...Third semiconductor layer (undoped Iny Ga1-y
As) 28... Fourth semiconductor layer (undoped Alx Ga1-x
As) 29...Contact layer (n+ type InGaAs) 30...Drain electrode 31...Source electrode 33...Gate electrode

Claims (1)

[Claims] 1. A first semiconductor layer made of a material with excellent electron transport properties that does not contain any impurities or contains impurities at a low concentration, and an n-type impurity that has a crystal structure that is approximately lattice-matched to the first semiconductor layer. InyGa1 containing a high concentration of In and having an In composition ratio Y of 0 or more and 0.35 or less
-y A first channel layer made of As, and a second semiconductor made of a material with excellent electron transport properties that does not contain any impurities or contains impurities at a low concentration and has a crystal structure that is approximately lattice matched to the first channel layer. The composition ratio Y of In formed thinly and containing a high concentration of n-type impurities having a crystal structure that is substantially lattice matched to the second semiconductor layer is 0 or more.
．． A second channel layer made of InyGa1-yAs with a particle size of 35 or less, and a material with excellent electron transport properties that does not contain any impurities or contains impurities at a low concentration and has a crystal structure that is almost lattice matched to the second channel layer. a third semiconductor layer consisting of a third semiconductor layer, and a composition ratio X of Al that forms a heterojunction with the third semiconductor layer and that does not contain any impurities or that contains a low concentration of impurities that make Schottky contact with the gate electrode is 0 or more and 0.3 or less. and a fourth semiconductor layer made of AlX Ga1-X As.